1. Field of the Invention
The present invention relates to a method and apparatus for slicing a workpiece and more particularly, relates to a method and apparatus for slicing a semiconductor ingot into warpage-free wafers by preventing deflection of the inner cutting edge of an annular saw blade during the slicing.
2. Related Prior Art
In general, the slicing of a workpiece such as a semiconductor ingot is carried out by rotating an annular saw blade having a cutting edge at the inner periphery thereof at a high speed, while supplying a coolant to the cutting edge. The coolant is used for purposes of cooling of the cutting edge of a saw blade engaged in slicing, and removal of the workpiece powders accumulated on the cutting edge. The coolant covers both surfaces of the rotating saw blade including the cutting edge in a thin film and is removed away by a centrifugal force.
Referring to FIG. 6, in a slicing operation, cutting surfaces S.sub.1, S.sub.2 and S.sub.3 of a semiconductor ingot 21 should preferably be parallel to each other and if so, high-quality wafers with no warpage are obtainable.
In order to realize the parallelism of the cutting surfaces S.sub.1, S.sub.2 and S.sub.3, the cutting edge of a saw blade is required to advance straight through the ingot 21 during slicing.
In the traditional methods and apparatuses, however, the cutting edge 1a of a saw blade 1 usually deviates to the direction of an ingot 21 as shown in FIG. 7 or, to the contrary, to the direction of a wafer 22 (not shown). As a result, as shown in FIG. 8, a cutting surface in the ingot 21 obliquely intersects the central axis of the ingot 21 and forms an inclined plane Sa, or a convexly or concavely curved surface Sb toward the wafer. In an extreme case, the cutting surface becomes a wavy surface Sc.
To sum up, it is difficult to obtain a warpage-free wafer and with much warpage in sliced wafers an yield of semiconductor devices fabricated thereon is reduced, which has been a problem.
A disclosure for slicing is made in a published document of unexamined Japanese Patent Application No. Hei 4-216013, which invention is entitled a cutting method for a slicing machine. In the disclosure, a slicing machine using an annular saw blade is shown, wherein air-nozzles are disposed near coolant nozzles and compressed air from the air nozzles removes unnecessary portion of the coolant on the blade which portion has no effect on the cutting.
According to the invention, dulling of the cutting edge of a saw blade is prevented effectively and a cleaning effect on the base metal portion of the blade is hightened.
The inventors of the present invention has confirmed, however, that such a method as above mentioned still has a difficulty in obtaining a warpage-free wafer.
One of the factors which causes deflection of the cutting edge of a saw blade is the difference in cutting sharpness between the A and B sides of the saw blade, wherein the A side surface faces an ingot and the B side surface faces a wafer being produced. The saw blade deflects to the side having better cutting sharpness. A deflection of the blade 1 is shown in FIG. 7 which deflection occurs when the sharpness on the side facing the ingot 21 is better, as an example of comparison in cutting sharpness between the two sides of the cutting edge 1a.
The inventors of the present invention have discovered that the deflection of the cutting edge of a saw blade is also caused by the imbalance in thickness between the coolant films on the A and B side surfaces of the base metal sheet 1b of the blade 1, said film being present in gaps both of which gaps are respectively formed on both side surfaces of the blade 1 between each cutting surface of the ingot and the side surface which faces said cutting surface.
A reason seems to be explained as follows: The imbalance in thickness between the coolant films causes the significant imbalance in size of the gaps each between any of the exposed coolant surfaces and one of the new cutting surface of the ingot being cut or a wafer being produced, which latter cutting surface directly faces the former coolant surface. The speeds of the air streams in the gaps relative to the saw blade are changed according to the sizes of the gaps, said air streams in the gaps relatively occurring by the rotation of the saw blade, since the air film on the surface the blade is transported into a gap together with the rotating blade. With a narrower gap, the relative speed increases to strengthen a degree of negativity in pressure in the space of the gap. The saw blade is attracted to a cutting surface of the ingot the gap on which is narrower in size than that on the other cutting surface.
In the above mentioned, published document, the deflection of the cutting edge of a saw blade is not prevented, since the imbalance in thickness between the coolant films on the blade surfaces of A and B sides cannot be eliminated, though the coolant film can be respectively thinned to some extent.
In addition, the thickness of a coolant film cannot be thinned to null, but will strenuously remain on the blade surfaces at a minimum because of viscosity. It is impossible to perfectly exclude the influence of the coolant upon slicing.
Gaps between the both cutting surfaces of an ingot and the both side surfaces of the base metal sheet 1b of a saw blade usually fall in the range of 50 .mu.m to 100 .mu.m. The cutting edge 1a of a saw blade 1 is, in general, made by bonding diamond abrasive grains on a blade base metal sheet 1b with a bond matrix such as nickel. The cutting sharpness imbalance between both sides of the saw blade 1 is caused by imbalance in worn-out degree of grains and/or in accumulation degree of removed workpiece material on the cutting edge between the both side thereof.
A blade deflection by the imbalance in thickness between the coolant films on the A and B side surfaces of a saw blade will be more particularly explained below in reference to FIG. 7.
A coolant film Cin exists at a thickness on the blade in the gap between the upper surface of the base metal sheet 1b and the cutting surface Sin of an ingot 21. Another coolant film Cw exists at a thickness on the blade in the gap between the lower surface of the base metal sheet 1b and the cutting surface Sw on a wafer 22.
With a lager difference in thickness between both coolant films on the upper and lower surfaces of the blade, a difference in size between gaps each from any of the exposed surfaces of the coolants to the corresponding cutting surface of Sin or Sw is larger. When the gap on the Sin cutting surface is narrower than that on the Sw cutting surface, the cutting edge 1a is attracted to the direction of the ingot 21. To the contrary, when the gap on the Sw cutting surface is narrower than that on the Sin cutting surface, the cutting edge 1a is attracted to the direction of the wafer 22. In such a manner as above described, the deflection of the blade occurs by the resultant force from the combination of the forces based respectively on the thickness imbalance of a coolant and on the sharpness imbalance in cutting of a saw blade.
In order to achieve an equalization in cutting sharpness on both sides of a saw blade, for example, dressing is applied to the cutting edge 1a, where the dressing is to recover the sharpness of a cutting edge by actually cutting a piece of green carborundum or dresser with the blade and thereby refreshing the cutting edge surface. However, even with such dressing, it is impossible to perfectly prevent the deflection or deviation of the cutting edge of the blade relative to a reference position.